$\displaystyle { \left( -3 {1 \over 4} - {5 \over 8} \right) \cdot240= \left( {-13 \over 4} - {5 \over 8} \right) \cdot240= \left( {-26 \over 8} - {5 \over 8} \right) \cdot240 =}$ 

 $\displaystyle{ ={-31 \over 8} \cdot240=-31\cdot30=-930 }$